Solar cell and method for forming the same

This application is a U.S. National Phase Patent Application based on International Patent Application No. PCT/EP2022/085147, filed Dec. 9, 2022; which claims priority to GB Patent Application No. 2119060.8, filed Dec. 29, 2021. The above referenced applications are incorporated herein by reference in their entirety as if fully set forth herein.

The present disclosure relates to solar cells and methods for forming the same.

Solar modules for providing electrical energy from sunlight comprise an array of solar/photovoltaic cells, each comprising a multi-layer semiconductor structure arranged between one or more front and back electrodes.

The substrate typically forms a p-n junction with a minority charge carrier collector layer (i.e. one of the substrate and the minority charge carrier collector layers being an n-type material and the other being a p-type material), which facilitates the generation of an electric current in response to light incident on the solar cell.

The solar cell can also include a majority charge carrier collector layer arranged on an opposite portion of the substrate to the minority charge carrier collector layer and is configured to extract charge carriers from the substrate. The front electrode is electrically connected to the minority charge carrier collector layer (e.g. electron collector layer) and the back electrode is electrically connected to the majority charge carrier collector layer (e.g. hole collector layer).

The minority and majority charge carrier collector layers are typically formed of amorphous silicon (a-Si) whereas the substrate is formed of crystalline silicon (c-Si) to form a heterojunction technology (HJT) solar cell.

To maximise the efficiency of such solar cells, it is important to minimise the number of surface defects which can form at the interfaces between the different layers of the multilayer structure. Surface defects typically consist of strained, or unterminated, bonds (also referred to as dangling bonds), which form within the semiconductor layers. Charge carriers can recombine at surface defects, instead of being collected by the electrodes, which leads to a decrease in the photovoltaic conversion efficiency of the solar cell.

Accordingly, there is a need to reduce the prevalence of charge carrier recombination within such solar cells, whilst also improving the charge carrier transport properties at the interface between the silicon substrate and the minority and majority charge carrier collector layers.

According to a first aspect there is provided a method of manufacturing a solar cell, the method comprising providing a substrate, arranging a passivation region on a surface of the substrate and arranging a collector layer on a surface of the passivation region, the step of arranging the passivation region comprises; depositing a first passivation layer on the surface of the substrate using a first gas; and, depositing a second passivation layer onto the surface of the first passivation layer using a second gas; wherein the first and second gases each comprise hydrogen gas and a silicon-based gas, wherein the ratio of hydrogen gas to silicon-based gas of the second gas is up to 2.5 and at least 0.4 times the ratio of hydrogen gas to silicon-based gas of the first gas.

It will be understood that the ratio of hydrogen gas to silicon-based gas of a source gas (e.g. the first and second gases) may define a ‘hydrogen gas ratio’ of the source gas, as will be referred to herein. In this way, the hydrogen gas ratio of the second gas may be between at least 0.4 times (i.e. at least 40%) and up to 2.5 times (i.e. up to 250%) the hydrogen gas ratio of the first gas. Put another way, the ratio between the hydrogen gas ratio of the second gas and the hydrogen gas ratio of the first gas may be up to 2.5 and/or at least 0.4. In the exemplary situation where the hydrogen gas ratio of the second gas is 0.4 times the hydrogen gas ratio of the first gas, then it will be understood that the concentration of hydrogen gas (relative to silicon-based gas) of the second gas is 0.4 times the concentration of hydrogen gas (relative to silicon-based gas) of the first gas.

The hydrogen gas ratios of the source gases according to the present invention are advantageously configured to cause densification of the respective first and second passivation layers, which thereby improves the film quality of the passivation region, leading to an increase in the conversion efficiency of the solar cell.

It will be understood that the first and second passivation layers define separate passivation layers, which are deposited in separate deposition steps (e.g. a first deposition step and a second deposition step, respectively). For example, the first passivation layer may be deposited in a first deposition chamber, and the second passivation layer may be deposited in a second deposition chamber, which is different to the first deposition chamber.

It is known that the negative effects of charge carrier recombination within the semiconductor layers, and at the interfaces therebetween, can be reduced by passivating the defects formed. Typically, this may be achieved by providing a ‘passivation layer’ of intrinsic (i.e. non-doped) semiconductor material between the substrate and the collector layer (e.g. an electron or hole-collector layer).

Such a known passivation layer may form a dielectric coating on the silicon substrate which chemically neutralises the dangling surface bonds—i.e. ‘chemical passivation’. This type of chemical passivation can be enhanced by introducing additional passivating species, such as hydrogen, into the intrinsic material during the deposition of the passivation layer.

The presence of a single passivation layer, while beneficial, may also result in the formation of additional interfaces within the solar cell, e.g. between the passivation layer and the overlying collector layer, which can increase the number of defect sites. This problem can be exacerbated if the passivation layer is too thin, leading to increased charge carrier recombination within the solar cell. Conversely, if the passivation layer is too thick, then its presence within the solar cell structure can inhibit transportation of charge carriers towards the electrodes.

It is known to expose the passivation layer of the solar cell to a hydrogen plasma treatment in order reduce defects both in the bulk of the layer and at its interfaces with other layers of the solar cell. The charged hydrogen ions which are formed during a known hydrogen plasma treating process can only penetrate a short distance into the treated semiconductor layer and, thus, can provide only limited passivation of the underlying bulk material.

In some cases, hydrogen plasma etching (or treating) can cause non-uniform passivation across the surface of the treated semiconductor layers. This can be a particular issue when multiple silicon substrates are arranged within a deposition chamber. Over time, the effects of the hydrogen plasma etching can lead to different levels of etching being performed on the silicon substrates, depending on where they are positioned in the chamber. This variation in the etching performance can cause significant variations in the conversion characteristics, particularly in open circuit voltage (Voc), of the final solar cells.

It has been determined by the inventors of the present invention that by controlling the hydrogen gas ratio of the source gases (e.g. the first and second gases), then the density of the first and second passivation layers can be increased to provide increased passivation of the interfaces within the solar cell, such that the hydrogen plasma etching can be avoided.

In particular, by configuring the hydrogen gas ratio of the second passivation layer so as to be substantially within a range of 0.4 and 2.5 times the hydrogen gas ratio of the first passivation layer, the inventors have determined that the resulting bi-layered passivation region is formed with a greater density (e.g. comprising a less porous structure). The passivity of the bulk material of the passivation region provides a more uniform passivation across a length, width, and depth of the film. The overall effect of the resulting bi-layered passivation region (i.e. including the first and second passivation layers) is a solar cell which exhibits an increased fill-factor.

A consequence of the method of present invention is that it may result in a slight reduction in the deposition rate for the second passivation layer, compared with an equivalent passivation layer that is deposited using a gas which has a significantly lower hydrogen concentration (e.g. a hydrogen concentration which is at least an order of magnitude lower than that of the first gas). Using such a low hydrogen concentration gas can increase the speed of deposition, but it may also lead to a situation in which a hydrogen plasma treatment is needed to enhance the passivating properties of the resulting layer (e.g. by performing a plasma treatment prior to depositing the passivation layer). By contrast, the method according to the present invention is advantageously configured such that the hydrogen gas ratio of the second gas is such that it produces a highly passivating second passivation layer, which does not require a hydrogen plasma treatment. Thus, the increased passivation of the second layer more than offsets any potential reduction in deposition speed.

It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on”, “adjacent” or “opposite” to an element, it can be “directly on”, “directly adjacent” or “directly opposite” to that further element; alternatively, there may be one or more intervening elements present. In contrast, when an element is referred to as being “directly on”, “directly adjacent” or “directly opposite” another element, there are no intervening elements present.

Optional features will now be set out. These are applicable singly or in any combination with any aspect.

It will be appreciated that etching defines an at least partially subtractive manufacturing process, which entails the removal of material from the surface which is being etched. The hydrogen plasma etching may define a process which is at least partially subtractive but also at least partially additive because hydrogen atoms are deposited upon, and into, the layer which is being treated, e.g. by passivating dangling bonds which are present on the surface of the passivation layer. The amount of material removed by etching may be more than the amount of material added by etching. In this way, hydrogen plasma etching may be defined as a surface passivation treatment.

The method of arranging the passivation region may comprise not etching at least one of the first and second passivation layers with a hydrogen plasma. In this way, neither the first nor the second passivation layer is treated, or etched, with a hydrogen plasma during the method.

In embodiments, the method may comprise not etching the first and/or second passivation layers with any form of plasma. The method may further comprise not treating the first and/or second passivation layers with a hydrogen plasma. Furthermore, the method may comprise not treating the first and/or second passivation layers with any form of surface passivation treatment.

The ratio of hydrogen gas to silicon-based gas of the second gas may be between 0.75 and 1.5 times the ratio of hydrogen gas to silicon-based gas of the first gas. In embodiments, the method may comprise configuring the first and second gases such that their respective ratios of hydrogen gas and silicon-based gas are substantially the same. In this way, the first gas is configured such that the level of hydrogen gas which is introduced into the deposition chamber during the deposition of the first passivation layer is substantially the same as that which is present during deposition of the second passivation layer. Accordingly, the resulting first and second passivation layers may be configured with substantially the same hydrogen gas ratio (e.g. percentage or proportion of hydrogen), which increases the density of the passivation region and thereby enhances the passivation of the interface between the passivation region (e.g. the first and second passivation layers) and the collector layer which is deposited thereon.

The method may comprise configuring the first gas such that the ratio of hydrogen gas to silicon-based gas is up to 50 and/or at least 20. In this way, the first gas may be configured such that there is up to fifty times, and/or at least twenty times, the amount of hydrogen gas than the amount of silicon-based gas present in the deposition chamber during the deposition of the first passivation layer.

In embodiments, the first gas may be configured such that the ratio of hydrogen gas to silicon-based gas is up 35 and/or at least 25. The method may comprise configuring the second gas such that the ratio of hydrogen gas to silicon-based gas is up to 50 and/or at least 20. In embodiments, the second gas may be configured such that the ratio of hydrogen gas to silicon-based gas is up 35 and/or at least 25. At least one, or each, of the first and second gases may be configured such that the ratio of hydrogen gas to silicon-based gas is approximately 32.

In embodiments the second passivation layer may be deposited directly onto the first passivation layer such that there is no interposing layer, or element, arranged therebetween. This contrasts with the situation in which a passivation layer is treated with a hydrogen plasma, wherein the hydrogen plasma may cause the formation, or build-up, of hydrogen atoms on the exposed surface of the passivation layer. Accordingly, it will be appreciated that any subsequent layer which is deposited onto the plasma treated surface would not be deposited ‘directly’ onto the passivation layer, but instead would be deposited onto the build-up of hydrogen atoms which are formed thereon by the hydrogen plasma treatment.

In embodiments, the passivation region may comprise a third passivation layer interposed between the first passivation layer and the substrate. The method of manufacturing the solar cell may comprise depositing the third passivation layer onto the surface of the substrate using a third gas comprising hydrogen gas and a silicon-based gas. The method may comprise depositing the third passivation layer prior to depositing the first and second passivation layers.

The ratio of hydrogen gas to silicon-based gas of the third gas may be up to 0.1 times the ratio of hydrogen gas to silicon-based gas of at least one of the first and second gases. In embodiments the ratio of hydrogen gas to silicon-based gas of the third gas may be up to 0.01 times the ratio of hydrogen gas to silicon-based gas of at least one of the first and second gases. The method may comprise configuring the third gas such that the ratio of hydrogen gas to silicon-based gas is up to 1. In embodiments, the third gas may be configured such that the ratio of hydrogen gas to silicon-based gas is approximately 0 (e.g. 0), i.e. substantially pure silicon-based gas. Put another way, the third gas may contain only trace amounts of hydrogen gas.

It will be understood that the collector layer defines a charge carrier collector layer of the solar cell. Accordingly, the collector layer may be configured to collect charge carriers (e.g. electrons and holes) which are generated due to the absorption of incident light, when the solar cell is in use. Depending on how the solar cell's components are configured (e.g. the conductivity type of the collector layer and substrate) the electrons and holes may define minority or majority charge carriers, when the solar cell is in operation.

The passivation region and the collector layer may, together, define at least part of a layered structure which is arranged on a surface of the substrate. For example, the layered structure may be arranged opposite a back surface of the substrate (i.e. a back layered structure), which is configured to not face a radiative source (e.g. the sun), when the solar cell is in use. Alternatively, the layered structure may be arranged on the front surface of the substrate, upon which light from a radiative source is incident during normal use (i.e. a front layered structure).

It will be appreciated that the layered structure may comprise one or more collector layer(s) without diverting from the scope of the present invention.

When the layered structure is a back layered structure it may comprise a back-collector layer and a back-passivation region, comprising a back first and second passivation layers. Similarly, when the layered structure is a front layered structure, it may comprise a front collector layer and a front-passivation region, comprising front first and second passivation layers.

The method of manufacturing the solar cell may comprise arranging the substrate in a deposition chamber and then depositing at least one or each of the regions and layers of the solar cell onto the substrate using a vapour deposition process in which one or more gases are introduced into the deposition chamber so as to form chemical species which are deposited onto a surface of the substrate. The deposition process may be a chemical vapour deposition process (CVD), e.g. a plasma enhanced chemical vapour deposition process (PECVD), as would be understood by the skilled person. Each of the layers of the passivation region, and/or the collector layer, may be deposited using the same deposition method as a part of a single continuous process. It will be understood that although they are deposited using a similar deposition method, at least one (or each) of the passivation layers (and/or collector layers) may be deposited in different deposition steps, and/or in different deposition chambers.

The method of depositing the passivation region may comprise first depositing the first passivation layer onto a surface of the substrate and then depositing the second passivation layer onto an exposed surface of the first passivation layer. In this way, the first and second passivation layers may be deposited sequentially to form the passivation region.

The method of arranging the collector layer may comprise, once the passivation region has been formed on the substrate, depositing the collector layer onto an exposed surface of the passivation region (e.g. the second passivation layer), such that the passivation region is interposed between the substrate and the collector layer. The method of depositing the collector layer may comprise depositing the collector layer on the surface of the passivation region using a fourth gas. The fourth gas may be different to at least one, or each, of the first, second and third gases.

At least one, or each, of the gases used to deposit the layers of the solar cell may be comprised of a plurality of gas species, or constituent gases, (e.g. gases having different chemical compositions). For example, the first gas may define a first gas mixture including hydrogen gas and silicon-based gas. For example, the second gas may define a second gas mixture including hydrogen gas and silicon-based gas.

Each of the gas mixtures may be formed by mixing a plurality of constituent gases before they are introduced into the deposition chamber. Alternatively, the constituent gases may be mixed within the deposition chamber. At least one, or each, of the first, second and third gases, i.e. the source gases, may comprise a silicon containing gas, such as SiHor SiH. At least one, or each, of the source gases may also comprise hydrogen gas (H).

The method may comprise controlling at least one parameter of the deposition process to determine the structural and/or chemical composition of at least one of the layers of the passivation region, and/or the collector layer.

The at least one parameter may comprise at least one of a gas flow rate, a gas pressure, a temperature of the deposition chamber, a temperature of the deposition chamber, and a power density of a plasma enhanced deposition process. In particular, the least one deposition parameter may determine the ratio of hydrogen gas to silicon-based gas of the first and second passivation gases. For example, the ratio between the hydrogen gas and the silicon-based gas may be determined by the relative volume fraction of the respective gases that are introduced into the deposition chamber.

In situations where the deposition process comprises a plasma enhanced vapour deposition process, a parameter of the deposition process may define the power density of the plasma which is induced during the deposition of the solar cell layers. Alternatively, this parameter may be defined as the radio frequency (RF) power which is used to form the plasma in the deposition chamber.

According to an exemplary method, the at least one (or each) deposition parameter(s) associated with the first passivation layer may be substantially the same as the corresponding at least one (or each) deposition parameter(s) of the second passivation layer.

The method may comprise depositing the first and second passivation layers with substantially the same gas pressure and power density deposition parameters. In embodiments, the gas pressure may be approximately 1.9 mbar and the power density may be approximately 21 mW/cm. The hydrogen ratio of the third gas may be substantially zero. Put another way, the third gas may comprise no hydrogen gas, and instead may be comprised of silicon-based gas only. The gas pressure and power density deposition parameters of the third passivation layer may be substantially different to that of the first and second passivation layers. In embodiments, the gas pressure may be approximately 1.2 mbar and the power density may be approximately 60 mW/cm.

Each of the layers may be configured with a width, a length, and a depth. The width and length of each layer may be measured in perpendicular directions that are aligned with the surface of the substrate upon which they are arranged. For each layer, its width and length may be substantially greater than its depth, which may be measured in a direction that is perpendicular to the substrate surface.

The method may comprise configuring the passivation region with a depth of less than 25 nm, optionally at least 5 nm. The method may comprise configuring the first passivation layer with a depth of less than 8 nm and at least 2 nm. The method may comprise configuring the second passivation layer with a depth of less than 9 nm and at least 3 nm. According to an exemplary arrangement, the first passivation layer may comprise a depth of approximately 5 nm, the second passivation layer may comprise a depth of approximately 6 nm. The method may comprise configuring the third passivation layer with a depth of less than 5 nm and at least 2 nm, optionally at least 3 nm. The method may comprise configuring the collector layer with a depth of less than 30 nm and at least 5 nm.

The at least one parameter of the deposition process may be configured to determine other aspects of the passivation region layers and/or collector layer. For example, a deposition rate for each of the layers may be determined by controlling one of the deposition parameters. The deposition rate may be defined as the depth of the layer (e.g. as measured in nanometres) which is deposited onto a smooth surface over time (e.g. as measured in seconds). The deposition rate of the first passivation layer may be substantially the same as the deposition rate of the second passivation layer. The deposition rate of the first and second passivation layers may be at least 0.03 nm/sec.

As described above, the method may comprise controlling the at least one parameter of the deposition process to determine the chemical composition of at least one of the layers of the passivation region, and/or the collector layer.

The method may comprise configuring at least one, or each, of the passivation layers and the collector layer such that they are substantially formed of a semiconductor material. At least one, or each, of the layers may be formed of amorphous silicon. It will be appreciated that the term ‘amorphous silicon’ is used herein to refer to a silicon-based amorphous semiconductor material. Examples of the silicon-based material include silicon alloys such as silicon carbide, silicon nitride and silicon germanium in addition to silicon. As such, each of the layers may be comprised of additional elements in combination with silicon. This is contrast to the substrate which may be formed of a substantially crystalline silicon material, e.g. monocrystalline or polycrystalline silicon.